In processing steel or other metals, large rolls in a stack are used to compress the metal to reduce the thickness of the material going through the mill. A screwdown, such as a rolling mill screw, is used in the mill to press the rolls in the stack tighter together to achieve more reduction in the thickness of the processed metal. Precise determination of the position of the rolling mill screw is required to control the thickness of the rolled material leaving the mill in order to ensure that it will be that which is desired.
One method used for determining the position of the mill screw is through the use of a stack of bowed flat springs, each set of springs being rotated 90 degrees from the preceding set. One end of the spring stack is connected to the mill screw, while the opposite end is connected to a potentiometer. As the mill screw is turned to tighten the rolls in the stack, the spring stack is compressed as it turns. The position of the mill screw is then determined by the changed readings of the potentiometer. However, this design suffers from lack of precision in transmitting the information corresponding to the rotary rotation of the mill screw through the spring stack to the potentiometer which is fixed. Additionally, the spring stack and potentiometer lack the ruggedness required for sustained use in a metal processing application.
Another method for determining the position of the mill screw is to use a spline to follow the rotation of the mill screw. However, splines experience backlash which prevents the accurate measurement of the rotation of the mill screw. Backlash is the relative motion of mechanical parts caused by looseness, such as between a gear tooth and a gear space. A spline following the rotation of the mill screw will experience backlash such that the spline will follow the screw as it rotates in one direction, but if the direction is reversed, there will be a delay before the spline will again follow the rotation of the screw. The rotation of the screw without the spline following it results in errors in the measurement of the rotation of the mill screw. Therefore, accurate measurement of the rotation of the mill screw cannot be obtained through use of a spline that experiences backlash.
Thus, until the development of the present invention, no one has provided a means by which the accurate rotary motion of a rolling mill screw can be measured precisely and reliably. PG,4